1. Field of the Invention
The present invention relates to an electrode for a PTC thermistor and a production method thereof, and a PTC thermistor using the same.
2. Description of the Prior Art
In recent years, overcurrent protective devices such a resettable fuse have been used increasingly to protect lithium electric cell batteries, interfaces of digital electronic equipment.
A thermistor having a positive temperature coefficient (hereinafter, referred to as PTC thermistor) is known as one of the overcurrent protective devices. The PTC thermistor includes a conductive polymer in which conductive particles are filled in a crystalline polymer, and a pair of electrodes that are arranged on both surfaces of the conductive polymer. When an overcurrent occurs in the PTC thermistor, the temperature of the conductive polymer increases to a temperature in the vicinity of the melting point of the crystalline polymer due to self-heating, so that the crystalline polymer expands in volume. When the crystalline polymer expands in the vicinity of the melting point, the conductive paths of the conductive particles in the crystalline polymer are broken. As a result, the resistance between the electrodes becomes high, and the current flowing through the PTC thermistor attenuates. In this manner, the PTC thermistor attenuates the overcurrent.
In the PTC thermistor, when the adhesion between the electrodes and the conductive polymer is weak, the following problem occurs. When an overcurrent is applied repeatedly, the electrical resistance between the electrodes and the conductive polymer becomes large. As a result, the reliability is reduced, or the PTC thermistor no longer operates properly as a device. Therefore, a strong adhesion is demanded between metal foils as the electrodes and the conductive polymer.
To enhance the adhesion between the metal foils and the conductor polymer, a PTC thermistor using a metal foil having roughness formed by electrodeposition is reported (Japanese Patent Publication No. 2788968). This patent discloses a method for forming a microrough surface by exposing a metal foil to an electrolyte, followed by electrodeposition.
However, the conventional method for forming roughness on a surface of a metal foil by electrodeposition as described above has a problem in that the adhesion between the conductive polymer, which is a resin, and the metal foils is not necessarily sufficient. For this reason, the conventional PTC thermistor as described above has a problem in that repeated application of overcurrent increases the change ratio in resistance.
Furthermore, since an electrodeposition treatment requires a long period of time, the production cost is high. In addition, it is difficult to control a plating solution during an electrodeposition treatment, so that a metal foil with a stable quality cannot be obtained.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an electrode for a PTC thermistor that has a large adhesion to the conductive polymer and can be produced easily, and a production method thereof, and a PTC thermistor using the same.
An electrode for a PTC thermistor of the present invention includes a base layer having electrical conductivity and a sintered layer formed on the base layer. The sintered layer is formed by sintering a conductive powder and has electrical conductivity, and has roughness on a surface thereof The present invention can provide an electrode for a PTC thermistor that has a large adhesion to the conductive polymer and can be produced easily.
In the electrode for a PTC thermistor of the present invention, preferably, the center line average roughness Ra of the sintered layer is from 0.5 xcexcm to 20 xcexcm. This embodiment can provide an electrode for a PTC thermistor that has a particularly large adhesion to the conductive polymer.
In the electrode for a PTC thermistor of the present invention, preferably, the average particle diameter of the conductive powder is from 0.1 xcexcm to 50 xcexcm. This embodiment can provide an electrode for a PTC thermistor that has a particularly large adhesion to the conductive polymer.
In the electrode for a PTC thermistor of the present invention, preferably, a metal coating is formed on the surfaces of the particles of the conductive powder. This embodiment can provide an electrode for a PTC thermistor where the sintered layer can be formed easily.
In the electrode for a PTC thermistor of the present invention, the base layer is formed of a metallic material, and the metal coating may be formed of the same material as that of the base layer. In this embodiment, the diffusion speeds of the base layer and the conductive powder during sintering are equal. Therefore, the base layer and the conductive powder are bonded by sintering in a short time. Thus, this embodiment can provide an electrode for a PTC thermistor where the sintered layer can be formed particularly easily.
In the electrode for a PTC thermistor of the present invention, the base layer may be formed of a metallic material, and the metal coating may be formed of a material having a melting point lower than that of the base layer. Since the conductive powder can be sintered at low temperatures, this embodiment can provide an electrode for a PTC thermistor where the sintered layer can be formed particularly easily.
In the electrode for a PTC thermistor of the present invention, preferably, the conductive powder includes a powder containing conductive particles that are linked one after another. This embodiment can provide an electrode for a PTC thermistor having a particularly large adhesion to the conductive polymer, because the volume of voids in the sintered layer can be increased.
In the electrode for a PTC thermistor of the present invention, preferably, the conductive powder includes a first powder having electrical conductivity and a second powder having electrical conductivity. The average particle diameter of the first powder is at least twice the average particle diameter of the second powder. This embodiment can provide an electrode for a PTC thermistor where the sintered layer can be formed particularly easily, because the second powder having a small particle diameter is arranged in voids formed by the first powder having a large particle diameter.
In the electrode for a PTC thermistor of the present invention, preferably, the content of the second powder in the conductive powder is not more than 60 wt %. This embodiment can provide an electrode for a PTC thermistor that has a sufficient adhesion to the conductive polymer and where the sintered layer can be formed particularly easily, because the first powder having a large particle diameter ensures the adhesion to the conductive polymer.
Preferably, the electrode for a PTC thermistor of the present invention further includes a metal film between the base layer and the sintered layer. This embodiment can provide an electrode for a PTC thermistor where the base layer and the conductive powder can be bonded by sintering easily.
In the electrode for a PTC thermistor of the present invention, preferably, the metal film includes at least one element selected from the group consisting of nickel, copper, silver, gold, palladium, titanium, zinc, molybdenum, tungsten, manganese, lead, chromium, platinum, tin, cobalt and indium. This embodiment can provide an electrode for a PTC thermistor where the base layer and the conductive powder can be bonded by sintering particularly easily.
In the electrode for a PTC thermistor of the present invention, preferably, the base layer has roughness on a surface thereof. This embodiment can provide an electrode for a PTC thermistor having a large adhesion between the base layer and the sintered layer.
In the electrode for a PTC thermistor of the present invention, preferably, the sintered layer includes a first sintered layer and a second sintered layer laminated in this order from the side of the base layer. The first sintered layer is formed by sintering a conductive powder with an average particle diameter of 0.1 xcexcm to 1 xcexcm. The second sintered layer is formed by sintering a conductive powder with an average particle diameter of not less than 1 xcexcm. In this embodiment, the first sintered layer increases adhesion between the base layer and the sintered layer, and the second sintered layer increases adhesion between the sintered layer and the conductive polymer. Thus, this embodiment can provide an electrode for a PTC thermistor having large adhesion both between the base layer and the sintered layer and between the sintered layer and the conductive polymer.
In the electrode for a PTC thermistor of the present invention, preferably, the conductive powder is formed of a metallic material comprising at least one element selected from the group consisting of iron, nickel, copper, silver, gold, palladium, zinc, molybdenum, tungsten, manganese, lead, chromium, platinum, tin, cobalt, indium and titanium. This embodiment can provide an electrode for a PTC thermistor having an excellent electrical conductivity, because the conductive powder has a good electrical conductivity so that the contact resistance with the conductive polymer can be small.
In the electrode for a PTC thermistor of the present invention, preferably, the base layer is formed of a metallic material comprising at least one element selected from the group consisting of iron, copper and nickel. This embodiment can provide an electrode for a PTC thermistor having a particularly excellent electrical conductivity.
A method for producing an electrode for a PTC thermistor of the present invention includes the first step of coating a surface of a base layer having electrical conductivity with a paste containing a conductive powder; and the second step of forming a sintered layer where the conductive powder is sintered by subjecting the paste to a heat treatment. The method for producing an electrode for a PTC thermistor of the present invention allows the electrode for a PTC thermistor of the present invention to be produced easily. Furthermore, the method for producing an electrode for a PTC thermistor of the present invention allows the sintered layers having various center line average roughnesses Ra to be formed easily by changing the particle diameter or shape of the conductive powder contained in the paste, or the film thickness of the sintered layer.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the average particle diameter of the conductive powder is from 0.1 xcexcm to 50 xcexcm. This embodiment makes it possible to produce an electrode for a PTC thermistor having a particularly large adhesion to the conductive polymer.
Preferably, the method for producing an electrode for a PTC thermistor of the present invention further includes the step of forming a metal film on a surface of the base layer before the first step. This embodiment makes it possible to bond the base layer and the conductive powder by sintering particularly easily.
Preferably, the method for producing an electrode for a PTC thermistor of the present invention further includes the step of forming roughness on a surface of the base layer before the first step. This embodiment makes it possible to produce an electrode for a PTC thermistor having a large adhesion between the base layer and the sintered layer.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the average particle diameter of the conductive powder is from 0.1 xcexcm to 1 xcexcm. The method further includes a third step of coating the sintered layer with a paste containing a conductive powder with an average particle diameter of not less than hum and performing a heat treatment so as to form another sintered layer laminated on the sintered layer. This embodiment makes it possible to laminate a dense sintered layer and a sintered layer having a large number of voids in this order from the side of the base layer, so that an electrode for a PTC thermistor including a sintered layer having large adhesion to the base layer and the conductive polymer can be produced.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the first step further includes the step of pressing the paste with which the base layer is coated into a sheet and drying the paste after the coating step. This embodiment makes it possible to bond the base layer and the conductive powder by sintering easily.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the heat treatment is performed in a reducing atmosphere. This embodiment makes it possible to form a sintered layer whose surface is not oxidized. Thus, an electrode for a PTC thermistor having a particularly small change ratio in resistance can be produced by using the electrode for a PTC thermistor including such a sintered layer.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the conductive powder is formed of a metallic material comprising at least one element selected from the group consisting of iron, nickel, copper, silver, gold, palladium, zinc, chromium, platinum, tin, cobalt, indium and titanium.
In the method for producing an electrode for a PTC thermistor of the present invention, preferably, the base layer is formed of a metallic material comprising at least one element selected from the group consisting of iron, copper and nickel. This embodiment makes it possible to produce an electrode for a PTC thermistor having an excellent electrical conductivity.
A PTC thermistor of the present invention includes at least one pair of electrodes and a conductive polymer arranged between the pair of electrodes (a plurality of pairs of electrodes may be included). The electrodes include a base layer having electrical conductivity and a sintered layer formed on a surface of the base layer on the side of the conductive polymer. The sintered layer is formed by sintering a conductive powder and has electrical conductivity, and has roughness on a surface thereof. This embodiment can provide a PTC thermistor having a small change ratio in resistance when an overcurrent is applied repeatedly, because the adhesion between the electrodes and the conductive polymer is large.
In the PTC thermistor of the present invention, preferably, the center line average roughness Ra of the sintered layer is from 0.51 xcexcm to 20 xcexcm. This embodiment can provide a PTC thermistor having a particularly small change ratio in resistance when an overcurrent is applied repeatedly, because the adhesion between the electrodes and the conductive polymer is particularly large.
In the PTC thermistor of the present invention, preferably, the average particle diameter of the conductive powder is from 0.1 xcexcm to 50 xcexcm. This embodiment can provide a PTC thermistor having a still more particularly small change ratio in resistance when an overcurrent is applied repeatedly.
In the PTC thermistor of the present invention, preferably, the conductive powder is formed of a metallic material comprising at least one element selected from the group consisting of iron, nickel, copper, silver, gold, palladium, zinc, chromium, platinum, tin, cobalt, indium and titanium. This embodiment can provide a PTC thermistor having a small electrical resistance, because the conductive powder has a good electrical conductivity so that the contact resistance with the conductive polymer can be small.
In the PTC thermistor of the present invention, preferably, the base layer is formed of a metallic material comprising at least one element selected from the group consisting of iron, copper and nickel. This embodiment can provide a PTC thermistor having a small electrical resistance, because the base layer has a good electrical conductivity.
As described above, the electrode for a PTC thermistor of the present invention includes a base layer having electrical conductivity and a sintered layer formed on the base layer. The sintered layer is formed by sintering a conductive powder and has electrical conductivity. Therefore, the electrode for a PTC thermistor according to the present invention has large adhesion to the conductive polymer and can be produced easily.
Furthermore, the method for producing the electrode for a PTC thermistor of the present invention includes the first step of coating a surface of a base layer having electrical conductivity with a paste containing a conductive powder, and the second step of forming a sintered layer containing the conductive powder by heating the paste. Therefore, according to this method, the electrode for a PTC thermistor of the present invention can be produced easily. In particular, according to this method, the center line average roughness can be controlled easily by changing the particle shape or particle diameter of the conductive powder in the paste or the film thickness of the sintered layer.
The PTC thermistor of the present invention includes a pair of electrodes and a conductive polymer arranged between the pair of electrodes and is characterized by using the electrodes for a PTC thermistor of the present invention. Therefore, the PTC thermistor of the present invention has a large adhesion between the electrodes for a PTC thermistor and the conductive polymer and a small change in resistance even if an overcurrent is applied repeatedly.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.